The piezoelectric actuator which utilizes the reverse piezoelectric effect of a piezoelectric ceramic finds various uses in, for example, precision position setting, a printer and a camera shutter. The piezoelectric ceramic for use in the piezoelectric actuator is specifically required to have large value of the piezoelectric constant d, so that the displacement per voltage is large, and also to exhibit a high Curie point, so that the piezoelectric actuator can be used in a wide range of temperature.
A piezoelectric ceramic represented by the formula: EQU Pb[(Ni.sub.1-b Zn.sub.b).sub.1/3 Nb.sub.2/3 ]O.sub.3 -PbTiO.sub.3 -PbZrO.sub.3
is known as an example of the piezoelectric ceramic having relatively large value of the piezoelectric constant d and a relatively high Curie point. The above piezoelectric ceramic is described in, for example, Japanese Patent Publication No. 48(1973)-8434 and Japanese Patent Laid-open Publication No. 5(1993)-58729.
With respect to the above piezoelectric ceramic represented by the formula: EQU Pb[(Ni.sub.1-b Zn.sub.b).sub.1/3 Nb.sub.2/3 ]O.sub.3 -PbTiO.sub.3 -PbZrO.sub.3,
the less the content of Pb[(Ni.sub.1-b Zn.sub.b).sub.1/3 Nb.sub.2/3 ]O.sub.3 or the greater the value of b, the higher the Curie point but the smaller the value of the piezoelectric constant d.
When the piezoelectric actuator is used in, for example, a high-speed drive element employed in a printer or the like, a high frequency and a high voltage are applied, so that the temperature of the element is raised. When the temperature of the element rises to about the Curie point, the displacement is rapidly reduced. Further, when the temperature of the element exceeds the Curie point, the polarization is released, thereby no longer exhibiting any significant displacement.
Therefore, a piezoelectric actuator capable of stably exhibiting a large displacement, even when the temperature of the element is high, has been demanded in the art. Thus, the development of a piezoelectric ceramic having not only large value of piezoelectric constant d, but also a high Curie point, as a starting material of the piezoelectric actuator has been desired in the art.
The inventors have extensively and intensively studied the effects of the variety and proportion of added oxide on the composition of the piezoelectric ceramic based on Pb[(Ni.sub.1-b Zn.sub.b).sub.1/3 Nb.sub.2/3 ]O.sub.3 -PbTiO.sub.3 -PbZrO.sub.3 in which the content of Pb[(Ni.sub.1-b Zn.sub.b).sub.1/3 Nb.sub.2/3 ]O.sub.3 is relatively small and the value of b is large. As a result, the inventors have found a piezoelectric ceramic which exhibits large value of the piezoelectric constant d and a high Curie point, so that it is useful as a material of a piezoelectric actuator.